Ben Carter
The lit candle under a glass on a plate of water experiment is a well-known demonstration of several scientific principles. The experiment involves lighting a candle and placing it in water under a glass. As the flame burns out, the water rises into the glass. This phenomenon has sparked curiosity and debate among scientists and enthusiasts alike, with various factors such as pressure, temperature, oxygen depletion, and buoyancy playing a role in the outcome of the experiment.
| Characteristics | Values |
|---|---|
| What happens when a candle is lit and placed in water under a glass? | As the flame burns out, the water gets pushed up into the glass. |
| Why does the water get pushed up? | The pressure inside the glass is reduced while the pressure outside the glass (due to the atmosphere) stays constant. Since the outside pressure is greater than the inside pressure, the water gets pushed up into the glass. |
| Why does the pressure inside the glass decrease? | It’s mostly due to the chemical reaction between the wax and oxygen. |
| What is the chemical reaction? | The heat of the flame vaporizes the liquid wax (turns it into a hot gas), and starts to break down the hydrocarbons into molecules of hydrogen and carbon. These vaporized molecules are drawn up into the flame, where they react with oxygen from the air to create heat, light, water vapour (H2O) and carbon dioxide (CO2). |
| What happens when the oxygen is depleted? | The candle will go out and stop producing heat. |
| What happens when there is more than one candle? | The water rises higher. |
| What type of candle is used in the experiment? | Paraffin candle. |
| What is the effect of the water on the candle? | The water cools the outer walls of the candle, preventing them from melting, which leads to a decrease in weight over time, but not buoyancy. |
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What You'll Learn
The lit candle experiment
Firstly, place a lit candle in a basin of water. The candle should be placed in such a way that it barely floats on the water surface. The water will prevent the outer layer of wax from melting, so it does not burn away. As the candle burns, it will slowly hollow out the middle, reducing its weight but not its volume.
Next, cover the candle and the basin with a glass. The flame will eventually go out due to a lack of oxygen. The burning candle produces carbon dioxide and water vapour, which causes the glass to become foggy.
Once the flame goes out, the heat production stops, and the gas inside the glass will begin to cool and contract. This change in volume creates a negative pressure inside the glass, and to balance this, water is pushed up into the glass by the greater external air pressure.
The water level will rise to compensate for the reduced volume of air inside the glass. This experiment demonstrates that the air we breathe contains approximately 21% oxygen.
It is important to note that the thickness of the candle and the number of candles used can impact the results of the experiment. Thinner candles produce less heat, resulting in less noticeable expansion and contraction of the air inside the glass. Conversely, using multiple candles generates more heat and leads to a more noticeable volume change.
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Water gets sucked up
The burning candle and rising water experiment is a classic demonstration that you can try at home. To perform the experiment, you light a candle and place it in water, then cover it with a glass. As the flame goes out, the water is seemingly "sucked" up into the glass. However, the water isn't actually sucked up; instead, this phenomenon is caused by a change in pressure. As the candle burns, it consumes oxygen, reducing the pressure inside the glass. Meanwhile, the pressure outside the glass, due to the atmosphere, remains constant. Since the outside pressure is now greater than the inside pressure, the water is pushed up into the glass.
This experiment illustrates the physics and chemistry behind the beauty and light of a candle flame. The key to most fire reactions is the combination of oxygen in the air with some type of carbon. In the case of a candle, the heat of the flame vaporizes the liquid wax, turning it into a hot gas. This process breaks down the hydrocarbons in the wax into molecules of hydrogen and carbon. These vaporized molecules are drawn up into the flame, where they react with oxygen to create heat, light, water vapour, and carbon dioxide.
The oxygen-rich region of the flame, known as the blue zone, is where the hydrocarbon molecules vaporize and start to break apart into hydrogen and carbon atoms. The hydrogen reacts with oxygen to form water vapour, while some of the carbon burns to form carbon dioxide. As these gases rise, they are heated to extremely high temperatures, reaching approximately 1000 degrees Celsius. As the various forms of carbon continue to break down, small, hardened carbon particles (soot) begin to form.
The dark or orange/brown region of the flame has relatively little oxygen. Here, the carbon particles continue to heat up until they ignite and emit a full spectrum of visible light. Since the yellow portion of the spectrum is the most dominant when the carbon ignites, the human eye perceives the flame as yellowish. Additionally, the flame's elongated or teardrop shape is due to the convection currents created by the cycle of upward-moving air around the flame.
The experiment also presents an interesting paradox. As the candle burns in the water, the water cools the outer walls of the candle, preventing them from melting. This leads to a decrease in weight over time, which affects the candle's buoyancy. The buoyant force gradually overtakes the weight, slightly pushing the candle up and reducing the amount of displaced water. This dynamic equilibrium results in the candle's continued burning while floating on the water's surface.
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Oxygen depletion
The lit candle experiment, which involves placing a lit candle under a glass on a plate of water, is a well-known demonstration of oxygen depletion. The experiment showcases the chemical reaction between the candle wax and oxygen, where oxygen from the surrounding air combines with carbon and hydrogen in the wax to form carbon dioxide, water vapour, and heat. As the candle burns, it consumes oxygen, leading to a decrease in oxygen levels within the glass. This depletion of oxygen eventually causes the candle to extinguish, ceasing the production of heat.
The oxygen depletion within the glass results in a reduction of pressure compared to the atmospheric pressure outside. Consequently, the higher outside pressure pushes the water up into the glass, leading to a rise in the water level. This phenomenon is not solely due to oxygen depletion, as argued by some, but rather a combination of factors, including the chemical reaction, pressure changes, and the convection current created by the flame.
The rate at which the water rises in the glass can vary depending on the number and thickness of candles used. A single thin candle may result in a lower water level rise compared to using multiple candles, indicating a potential correlation between the number of candles and the rate of oxygen depletion. Additionally, the type of wax used can also influence the outcome, with paraffin candles exhibiting specific characteristics in this experiment.
It is important to note that the water's rise is not immediate and occurs only after the candle dims and goes out. This observation contradicts the sole oxygen depletion argument, as a steady rise in the water level would be expected if oxygen depletion were the only factor at play. Instead, the rise in water level is rapid at the end, suggesting that multiple factors are influencing the outcome.
The lit candle experiment, therefore, provides a visual representation of the role of oxygen in combustion and the complex interplay between chemistry and physics. While oxygen depletion is a critical aspect, it is not the sole factor driving the water's rise in the experiment. The experiment continues to intrigue scientists and students alike, offering a fascinating glimpse into the underlying principles of heat, light, and combustion.
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Candle buoyancy
Floating candles are designed to remain buoyant on water surfaces, creating a mesmerizing and tranquil atmosphere. They are often used for special events, such as weddings, holiday celebrations, and romantic dinners. These candles are crafted with a flat base and made from various types of wax, with paraffin and soy wax being popular choices. The density of the wax plays a crucial role in achieving buoyancy.
When creating floating candles, it is important to consider both aesthetics and functionality. Decorations can be added to enhance the visual appeal, but they should not compromise the candle's buoyancy or flame. Safety is also a key concern, as the open flame can ignite nearby materials. It is recommended to keep floating candles away from flammable objects and always extinguish them before leaving the room.
The process of making floating candles involves choosing the right wax, melting it at the ideal temperature, and securing a pre-tabbed wick in the centre of a floating candle mould. The wax type and ratio can affect the candle's buoyancy and burn time. For example, soy wax offers a more sustainable option, while paraffin wax may produce toxic fumes and excess soot.
Additionally, the choice of wick can impact the burn rate. A thinner wick can slow down the burning process, resulting in longer float time. Experimentation with different wax blends and wick sizes can help achieve the desired buoyancy and burn duration.
The "rising water experiment" with a burning candle demonstrates the principles of pressure and gas laws. When a lit candle inside a closed container runs out of oxygen, the flame goes out. As the oxygen in the air reacts with carbon from the candle, the volume of oxygen decreases, creating a pressure difference that pulls water up into the container. This experiment showcases the relationship between gas pressure, volume, temperature, and the number of molecules involved.
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The chemical reaction
As the flame heats the surrounding air, it starts to rise, creating a convection current. This movement of warm air upwards allows cooler air and oxygen to rush in at the bottom of the flame to replace it. The cycle of upward-moving air continues as the cooler air is heated and rises, maintaining the flame's teardrop shape. The hydrogen molecules that separated from the wax combine with the oxygen in the air to form water vapour.
Additionally, some of the carbon atoms from the wax combine with oxygen to form carbon dioxide (CO2). This combustion process releases energy in the form of heat and light. The heat radiates from the flame, melting more wax to sustain the combustion process until the fuel is depleted or the heat source is removed. The ratio of oxygen molecules to carbon dioxide molecules is important, as it affects the volume and pressure of the gas produced.
The presence of water in the experiment, such as in the famous "lit candle under a glass of water" experiment, introduces additional factors. The water can cool the outer walls of the candle, reducing its weight over time and affecting its buoyancy. The depletion of oxygen within the container also plays a role in the overall chemical reaction, as the limited oxygen supply will eventually be consumed, causing the candle to go out.
Overall, the chemical reaction of a candle in water involves the vaporization of wax, the breakdown of hydrocarbons, the formation of water vapour and carbon dioxide, and the release of heat and light energy. The interaction between the candle, oxygen, and water results in a dynamic and intriguing series of chemical transformations.
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Frequently asked questions
The lit candle under a glass of water experiment demonstrates how much oxygen is in the air. The water rises as the flame goes out, filling the glass and indicating that the air is about 21% oxygen.
The pressure inside the glass is reduced as the oxygen is depleted, while the pressure outside the glass remains constant. As a result, the water is pushed up into the glass due to the greater outside pressure.
The burning candle combines oxygen from the air with carbon and hydrogen in the wax to form CO2, H2O, and heat. As the candle consumes the limited oxygen inside the glass, it eventually goes out.
In the late 1990s, NASA conducted experiments to observe candle flames in microgravity. Unlike on Earth, where the flame takes an elongated or teardrop shape, a candle flame in microgravity is spherical.
